0.18 round 1+2 integration fixes

Two clippy issues surfaced when round 1 polish + round 2 FrameSink
migrations both landed on libfreemkv main:

- src/halt.rs: clippy::new_without_default fires when a public new()
  exists without Default. The polish pass dropped the derive thinking
  it was redundant — clippy disagrees, so add a manual impl that
  forwards to new(). Doc-comment notes why both exist.

- src/disc/read_error.rs:372: pre-existing
  assert_eq!(.., true) trips clippy::bool_assert_comparison. Pre-0.18
  precommits passed because that lint sat outside the gate; the
  round-2 commits brought enough new clippy surface that it now
  shows up. Trivial cleanup: assert!(...) instead of assert_eq!.

Single contributor: MattJackson.

src/disc/read_error.rs

@@ -369,6 +369,6 @@ mod tests {
         ctx.on_success();
         assert_eq!(ctx.consecutive_good, 1);
         assert_eq!(ctx.consecutive_failures, 0);
-        assert_eq!(*ctx.damage_window.last().unwrap(), true);
+        assert!(*ctx.damage_window.last().unwrap());
     }
 }